In the latter years there have been an increasing need for being able to install large subsea constructions from installation vessels, see for instance http://fuelfix.com/blog/2014/05/06/statoil-pursuing-huge-subsea-factories/, http://www.statoil.com/no/technologyinnovation/fielddevelopment/aboutsubsea/Pages/GullfaksVaatgasskompression.aspx and http://www.offshoreenergytoday.com/douglas-westwood-117b-to-be-spent-on-subsea-systems-by-2018/.
For submerging of heavy constructions from installation vessel it is common to use the work well (i.e. moonpool) of the installation vessel since such solutions usually reduce the requirement of comprehensive weight balancing. An example of submerging of loads through a center well is described in the article «Lowering and lifting operations through moonpools: Hydrodynamic investigations» by Sigve Håland, Marine Technology, June 2014 (http://www.diva-portal.org/smash/get/diva2:742177/FULLTEXT01.pdf). An important criteria for using the work well of the vessel is among other things that the available lifting equipment has the required lifting capacity for accomplishing the submerging operation.
However, this solution requires that the cross-section of the center well is larger than the cross-section of the particular construction, and thus a need for larger vessels.
As an alternative to the submerging of loads through the center well of the vessel, the patent publication WO 2010/109243 discloses a solution, wherein the load to be submerged is arranged on an appropriate barge, and wherein the barge is guided horizontally below the installation vessel to a position being beneath the installation vessel by use of a tender vessel. The load is then raised from the barge by use of a dedicated crane and the barge is guided away by said tender vessel. Consequently, the load hangs below the installation vessel and is ready for submerging to the seabed. This known solution is considered to be very expensive since it requires the use of barges having at least two side columns of large dimensions, and use of a tender vessel, both for guiding the barge with the load to the installation area and for positioning the load below the vessel. In addition, the positioning of the barge will be vulnerable for environmental forces, such as subsea currents. Regarding other alternatives for submerging loads, patent publication U.S. Pat. No. 3,987,638 may be mentioned. This publication describes a system which submerges loads from the water line to a position below the installation vessel by use of buoyancy elements and lines. This system will also be vulnerable to environmental forces during operation. In addition, the positioning of the load will not be very predictable due to the significant movability of the lines.
Thus, it is a goal of one or more embodiments of the present invention to manufacture a load handling system which shows a high degree of stability during submerging of large and/or heavy loads to the seabed, even during harsh weather conditions, such as tall waves or strong currents, and which at the same time avoids expensive modifications of, and/or additions to, commercially available equipment. Another purpose of one or more embodiments of the present invention is to develop a system which can handle large and/or heavy loads through the work well of the vessel even when the minimal ground area of the work well is too small for allowing the relevant constructions/loads to be submerged therethrough.